I'd bet a lot of money on K-State fans saying KU is their biggest rival and not ISU.
I'm sure they have a lot of the dorks like the many Cyclone/Hawkeye fans that pretend they hate KSU or Wisconsin more because they think it's some dig at the in state rival. In reality I'm sure KU and KSU are more intense about each other than any other school including when Missouri was still in Big 12.
That gets into the debate of disliking a team vs a true rival. I have a lot of programs ISU frequently plays that I strongly dislike but aren't actually rivals. Nebraska back in the day, Baylor, formerly Texas, soon to be Utah.
Kansas I do think has sort of become a true rival with ISU. Look at the all time hardware for Big 12 basketball. ISU is a distant second, but still second, and third is a distant third to ISU. Just the fact that we've gotten in their way more than most other schools in the conference even with our inconsistency means if they have a hoops rival it's us. We'll see how Houston and Arizona do long term and how they do with whomever follows Self.